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Membrane Materials: Organic v. Inorganic

Organic Membrane

Synder offers multiple polymer types for Ultrafiltration and Microfiltration organic membranes, including PES (polyethersulfone), PVDF (polyvinylidenedifluoride), and PAN (polyacrylonitrile). Membrane selection depends on a variety of factors, including the composition of the feed solution, operating parameters, application type, and separation goals. While organic and inorganic membranes have their own advantages and disadvantages, it is important to determine what type of membrane or polymer is most suitable for the application.

A majority of industrial membranes consist of synthetic or natural polymers; membranes with both types of polymers are known as organic membranes. Examples of synthetic polymers include polytetrafluoroethylene (Teflon PTFE), polyamide-imide (PAI), and polyvinylidenedifluoride (PVDF) while natural polymers include rubber, wool, and cellulose.

Artificial polymers are synthesized by the polymerization of a monomer or co-polymerization of 2 monomers. Polymerization has 3 configurations: linear chains such as polyethylene, branched chains such as polysulfone, and cross-linked structures such as phenol-formaldehyde. Linear-chained polymers are more soluble in organic solvents. They become pliable or moldable with temperature increase and are known as thermoplastic polymers. On the other hand, cross-linked polymers are almost insoluble in organic solvents. They do not soften with temperature increase and are known as thermosetting polymers.

Polymer selection must be based on compatibility with membrane fabrication technology and intended application use. For example, the polymer may require a low affinity toward the permeate, while other times it may need to withstand harsh cleaning conditions due to membrane fouling. Chain interactions, chain rigidity, functional group polarity, and stereoisomerism also need to be factored into polymer choice and organic membrane manufacturing.

Inorganic Membranes

Metallic membranes are made from sintering metal powders such as tungsten, palladium or stainless steel and then depositing them onto a porous substrate. The main use of metallic membranes is for hydrogen separation with palladium and its alloy being the primary choice of material. One major disadvantage for metallic membranes is surface poisoning effect.

Ceramic membranes consist of metal (aluminum or titanium) and non-metal (oxides, nitride, or carbide). They are generally used for highly acidic or basic environments due to inertness. The downside of ceramic membranes is the high sensitivity to temperature gradient, which leads to membrane cracking.

Zeolite membranes are used in highly-selective gas separation due to highly uniform pore size. This material also has a catalytic characteristic, which is beneficial for catalytic membrane reactor applications. Few downsides of zeolite membranes include relatively low gas flux and thicker layer requirements to prevent cracks and pinholes.

Some advantages that inorganic membrane possesses are high thermal and chemical stability, inertness to microbiological degradation, and ease of cleaning after fouling compared to organic counterparts. However, inorganic membranes tend to have higher capital costs due to specific thickness requirements needed to withstand pressure drop differences.